Gate-all-around devices having self-aligned capping between channel and backside power rail

ABSTRACT

A semiconductor device includes a first interconnect structure and multiple channel layers stacked over the first interconnect structure. A bottommost one of the multiple channel layers is thinner than rest of the multiple channel layers. The semiconductor device further includes a gate stack wrapping around each of the channel layers except a bottommost one of the channel layers; a source/drain feature adjoining the channel layers; a first conductive via connecting the first interconnect structure to a bottom of the source/drain feature; and a dielectric feature under the bottommost one of the channel layers and directly contacting the first conductive via.

PRIORITY

This is a continuation application of U.S. application Ser. No.17/218,503, filed Mar. 31, 2021, which claims the benefits to U.S.Provisional Application No. 63/024,167, filed May 13, 2020, each ofwhich is herein incorporated by reference in its entirety.

BACKGROUND

Conventionally, integrated circuits (IC) are built in a stacked-upfashion, having transistors at the lowest level and interconnect (viasand wires) on top of the transistors to provide connectivity to thetransistors. Power rails (e.g., metal lines for voltage sources andground planes) are also above the transistors and may be part of theinterconnect. As the integrated circuits continue to scale down, so dothe power rails. This inevitably leads to increased voltage drop acrossthe power rails, as well as increased power consumption of theintegrated circuits. Therefore, although existing approaches insemiconductor fabrication have been generally adequate for theirintended purposes, they have not been entirely satisfactory in allrespects. One area of interests is how to create semiconductor deviceswith backside power rails and to isolate backside power rails fromfrontside components such as metal gates.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIGS. 1A, 1B, 1C, and 1D show flow charts of a method of forming asemiconductor device with backside power rails and backside self-alignedvias, according to embodiments of the present disclosure.

FIGS. 2 and 3 illustrate cross-sectional views of a portion of asemiconductor device, according to some embodiments, in intermediatesteps of fabrication according to an embodiment of the method of FIGS.1A-1D.

FIGS. 4A, 5A, 6A, 7A, 8A, 9A, 10A, and 19A illustrate top views of aportion of a semiconductor device, according to some embodiments.

FIGS. 4B, 5B, 5C, 6B, 6C, 7B, 7C, 8B, 8C, 9B, 9C, 10B, 10C, 11, 12A,12B, 12C, 13A, 13B, 13C, 13D, 19B, 19C, 19D, 20B, 20C, 20D, and 21Billustrate cross-sectional views of a portion of a semiconductor device,according to some embodiments.

FIGS. 14, 15, 16, 17, 18, 20A, and 21A illustrate perspective views of aportion of a semiconductor device, according to some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. Still further, when anumber or a range of numbers is described with “about,” “approximate,”and the like, the term encompasses numbers that are within certainvariations (such as +/−10% or other variations) of the number described,in accordance with the knowledge of the skilled in the art in view ofthe specific technology disclosed herein, unless otherwise specified.For example, the term “about 5 nm” may encompass the dimension rangefrom 4.5 nm to 5.5 nm, 4.0 nm to 5.0 nm, etc.

This application generally relates to semiconductor structures andfabrication processes, and more particularly to semiconductor deviceswith backside power rails and backside self-aligned vias. As discussedabove, power rails in IC need further improvement in order to providethe needed performance boost as well as reducing power consumption. Anobject of the present disclosure includes providing power rails (orpower routings) on a back side (or backside) of a structure containingtransistors (such as gate-all-around (GAA) transistors) in addition toan interconnect structure (which may include power rails as well) on afront side (or frontside) of the structure. This increases the number ofmetal tracks available in the structure for directly connecting tosource/drain contacts and vias. It also increases the gate density forgreater device integration than existing structures without the backsidepower rails. The backside power rails may have wider dimension than thefirst level metal (M0) tracks on the frontside of the structure, whichbeneficially reduces the power rail resistance. The present disclosurealso provides structures and methods for isolating the backside powerrails from frontside components such as metal gates. The details of thestructure and fabrication methods of the present disclosure aredescribed below in conjunction with the accompanied drawings, whichillustrate a process of making a GAA device, according to someembodiments. A GAA device refers to a device having vertically-stackedhorizontally-oriented multi-channel transistors, such as nanowiretransistors and nanosheet transistors. GAA devices are promisingcandidates to take CMOS to the next stage of the roadmap due to theirbetter gate control ability, lower leakage current, and fully FinFETdevice layout compatibility. Those of ordinary skill in the art shouldappreciate that they may readily use the present disclosure as a basisfor designing or modifying other processes and structures for carryingout the same purposes and/or achieving the same advantages of theembodiments introduced herein.

FIGS. 1A, 1B, and 1D are a flow chart of a method 100 for fabricating asemiconductor device according to an embodiment of the presentdisclosure. FIGS. 1A, 1C, and 1D are a flow chart of the method 100 forfabricating a semiconductor device according to an alternativeembodiment of the present disclosure. Additional processing iscontemplated by the present disclosure. Additional operations can beprovided before, during, and after method 100, and some of theoperations described can be moved, replaced, or eliminated foradditional embodiments of method 100.

Method 100 is described below in conjunction with FIG. 2 through FIG.21B that illustrate various top, cross-sectional, and perspective viewsof a semiconductor device (or a semiconductor structure) 200 at varioussteps of fabrication according to the method 100, in accordance withsome embodiments. In some embodiments, the device 200 is a portion of anIC chip, a system on chip (SoC), or portion thereof, that includesvarious passive and active microelectronic devices such as resistors,capacitors, inductors, diodes, p-type field effect transistors (PFETs),n-type field effect transistors (NFETs), FinFET, nanosheet FETs,nanowire FETs, other types of multi-gate FETs, metal-oxide semiconductorfield effect transistors (MOSFETs), complementary metal-oxidesemiconductor (CMOS) transistors, bipolar junction transistors (BJTs),laterally diffused MOS (LDMOS) transistors, high voltage transistors,high frequency transistors, memory devices, other suitable components,or combinations thereof. FIGS. 2 through 21B have been simplified forthe sake of clarity to better understand the inventive concepts of thepresent disclosure. Additional features can be added in the device 200,and some of the features described below can be replaced, modified, oreliminated in other embodiments of the device 200.

At operation 102, the method 100 (FIG. 1A) forms a stack 205 of firstand second semiconductor layers over a substrate 201. The resultantstructure is shown in FIGS. 2 and 3 according to an embodiment.Particularly, FIG. 2 illustrates the substrate 201 in an embodiment, andFIG. 3 illustrates a stack 205 of semiconductor layers 210 and 215 in anembodiment. In the depicted embodiment, substrate 201 is asemiconductor-on-insulator substrate, such as a silicon-on-insulator(SOI) substrate, a silicon germanium-on-insulator (SGOI) substrate, or agermanium-on-insulator (GOI) substrate. Semiconductor-on-insulatorsubstrates can be fabricated using separation by implantation of oxygen(SIMOX), wafer bonding, and/or other suitable methods. In the depictedembodiment, the substrate 201 includes a semiconductor layer 204, aninsulator 203, and a carrier 202. In embodiments, the semiconductorlayer 204 can be silicon, silicon germanium, germanium, or othersuitable semiconductor. The semiconductor layer 204 may be doped in someembodiments or undoped in some alternative embodiments. In the presentembodiment, the semiconductor layer 204 functions as a seed layer forepitaxially growing the semiconductor layer stack 205. The carrier 202may be part of a silicon wafer and the insulator 203 may be siliconoxide. In an alternative embodiment, the substrate 201 is a bulk siliconsubstrate (i.e., including bulk single-crystalline silicon). Thesubstrate 201 may include other semiconductor materials in variousembodiment, such as germanium, silicon carbide, gallium arsenide,gallium phosphide, indium phosphide, indium arsenide, indium antimonide,SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, GaInAsP, or combinationsthereof.

The semiconductor layer stack 205 includes semiconductor layers 210 andsemiconductor layers 215 stacked vertically (e.g., along thez-direction) in an interleaving or alternating configuration from asurface of the substrate 201. In some embodiments, semiconductor layers210 and semiconductor layers 215 are epitaxially grown in the depictedinterleaving and alternating configuration. For example, a first one ofsemiconductor layers 210 is epitaxially grown on substrate, a first oneof semiconductor layers 215 is epitaxially grown on the first one ofsemiconductor layers 210, a second one of semiconductor layers 210 isepitaxially grown on the first one of semiconductor layers 215, and soon until semiconductor layers stack 205 has a desired number ofsemiconductor layers 210 and semiconductor layers 215. In someembodiments, epitaxial growth of semiconductor layers 210 andsemiconductor layers 215 is achieved by a molecular beam epitaxy (MBE)process, a chemical vapor deposition (CVD) process, a metalorganicchemical vapor deposition (MOCVD) process, other suitable epitaxialgrowth process, or combinations thereof.

A composition of semiconductor layers 210 is different than acomposition of semiconductor layers 215 to achieve etching selectivityand/or different oxidation rates during subsequent processing. Invarious embodiments, semiconductor layers 210 and semiconductor layers215 include different materials, different constituent atomicpercentages, different constituent weight percentages, and/or otherdifferent characteristics to achieve desired etching selectivity duringan etching process, such as an etching process implemented toselectively remove the semiconductor layers 210. For example, wheresemiconductor layers 210 include silicon germanium and semiconductorlayers 215 include silicon, a silicon etch rate of semiconductor layers215 is less than a silicon germanium etch rate of semiconductor layers210. In some embodiments, semiconductor layers 210 and semiconductorlayers 215 can include the same material but with different constituentatomic percentages to achieve the etching selectivity and/or differentoxidation rates. For example, semiconductor layers 210 and semiconductorlayers 215 can include silicon germanium, where semiconductor layers 210have a first silicon atomic percent and/or a first germanium atomicpercent and semiconductor layers 215 have a second, different siliconatomic percent and/or a second, different germanium atomic percent. Thepresent disclosure contemplates that semiconductor layers 210 andsemiconductor layers 215 include any combination of semiconductormaterials that can provide desired etching selectivity, desiredoxidation rate differences, and/or desired performance characteristics(e.g., materials that maximize current flow), including any of thesemiconductor materials disclosed herein.

As described further below, semiconductor layers 215 or portions thereofform channel regions of the device 200. In the depicted embodiment,semiconductor layer stack 205 includes four semiconductor layers 210 andfour semiconductor layers 215. However, the present disclosurecontemplates embodiments where semiconductor layer stack 205 includesmore or less semiconductor layers, for example, depending on a number ofchannels desired for the device 200 (e.g., a GAA transistor) and/ordesign requirements of the device 200. For example, semiconductor layerstack 205 can include two to ten semiconductor layers 210 and two to tensemiconductor layers 215. As will be discussed, the method 100 willprocess layers at both sides of the substrate 201. In the presentdisclosure, the side of the substrate 201 where the stack 205 resides isreferred to as the frontside and the side opposite the frontside isreferred to as the backside.

In the present embodiment, the bottommost layer of the semiconductorlayers 215 is thinner than other semiconductor layers 215. Thus, it islabeled as semiconductor layer 215′ to distinguish it from othersemiconductor layers 215 in the following discussion. Further, thebottommost layer of the semiconductor layers 210 is thinner than othersemiconductor layers 210. Thus, it is labeled as semiconductor layer210′ to distinguish it from other semiconductor layers 210 in thefollowing discussion. The semiconductor layers 215 each have a thicknesst1, the semiconductor layers 210 each have a thickness t2, thesemiconductor layer 215′ has a thickness t3, and the semiconductor layer210′ has a thickness t4. In the present embodiment, t1 is greater thant3 and t2 is greater than t4.

As will be discussed, each of the semiconductor layers 215 will be fullywrapped around (for example, at its top, bottom, and sidewalls) by ametal gate for excellent short channel control, but the semiconductorlayer 215′ is not fully wrapped around by the metal gate. For example,the metal gate is not disposed under at least a portion of thesemiconductor layer 215′. Thus, gate control of the semiconductor layer215′ may not be as good as that of the semiconductor layers 215.Therefore, making the semiconductor layer 215′ thinner than thesemiconductor layers 215 (i.e., t3<t1) reduces leakage current (due tosub-channel effects) through the semiconductor layer 215′. In someembodiments, the thickness t1 may be in a range of about 4 nm to about 6nm, the thicknesses t3 may be in a range of about 1 nm to about 4 nm,while it is maintained that t3 is smaller than t1. In some embodiments,a ratio of t3 to t1 is in a range of greater than 0.33 and less than 1.If the ratio is too small (such as less than 0.33), the layer 215′ maybe too thin in some instances to sustain various oxidation and etchingprocesses. Without the layer 215′, the uniformity among the channellayers 215 might be degraded in some instances.

As will be discussed, the semiconductor layers 210 and 210′ will beremoved at subsequent fabrication steps and one or more dielectricmaterials will be deposited into the space occupied by the semiconductorlayers 210 and 210′. In the present embodiment, the one or moredielectric materials will fully fill the space occupied by thesemiconductor layer 210′ but do not fully fill the space occupied by thesemiconductor layers 210. To achieve that objective, the thickness t2 isdesigned to be greater than the thickness t4. In some embodiment, thethickness t2 is greater than the thickness t4 by about 3 nm to about 6nm. If the difference is too small (e.g., if t2−t4<3 nm), it might bedifficult to control the thickness of the one or more dielectricmaterials during deposition and the process margin might be very smallin some instances, which might adversely affect the product yield. Ifthe difference is too large (e.g., if t2−t4>6 nm), it might make themetal gate (which surrounds the layers 215) unnecessarily tall and thedevice dimension unnecessarily large, which might adversely affectdevice integration density for some implementations. In someembodiments, the thickness t4 is designed to be about 2 nm to about 5nm, while t2 is designed to be greater than t4 by about 3 nm to about 6nm. As will be discussed, the thickness t4 determines a thickness of adielectric feature replacing the semiconductor layer 210′ and thedielectric feature functions to isolate backside power rails fromfrontside metal gates. If the thickness t4 is too small (such as lessthan 2 nm) or too large (such as more than 5 nm), it might be difficultto fully fill the space occupied by the layer 210′ with dielectricmaterial(s) or the dielectric feature might not provide sufficientisolation for TDDB (Time Dependent Dielectric Breakdown) performancepurposes. In some embodiments, a ratio of t4 to t2 is designed to be ina range of about 0.33 to about 0.6. Again, if this ratio is too small(such as less than 0.33) or too large (such as more than 0.6), it mightbe difficult to fully fill the space occupied by the layer 210′ withdielectric material(s) or the dielectric feature might not providesufficient isolation for TDDB performance purposes.

At operation 104, the method 100 (FIG. 1A) forms fins 218 by patterningthe stack 205 and the substrate 201 and forms isolation features 230adjacent to sidewalls of the fins 218. FIG. 4A illustrates a top view ofthe device 200 with fins 218 oriented lengthwise along the “x”direction. FIG. 4B illustrates a cross-sectional view of the device 200,in portion, along the B-B line in FIG. 4A. As illustrated in FIG. 4B,the fins 218 include the patterned stack 205 (having layers 210, 215,210′, and 215′) and patterned semiconductor layer 204. The fins 218 maybe patterned by any suitable method. For example, the fin 218 may bepatterned using one or more photolithography processes, includingdouble-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a sacrificial layer is formed over the stack 205 and patterned using aphotolithography process. Spacers are formed alongside the patternedsacrificial layer using a self-aligned process. The sacrificial layer isthen removed, and the remaining spacers, or mandrels, may then be usedas a masking element for patterning the fins 218. For example, themasking element may be used for etching recesses into the stack 205 andthe substrate 201, leaving the fins 218 on the substrate 201. Theetching process may include dry etching, wet etching, reactive ionetching (RIE), and/or other suitable processes. For example, a dryetching process may implement an oxygen-containing gas, afluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), achlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), abromine-containing gas (e.g., HBr and/or CHBr₃), an iodine-containinggas, other suitable gases and/or plasmas, and/or combinations thereof.For example, a wet etching process may comprise etching in dilutedhydrofluoric acid (DHF); potassium hydroxide (KOH) solution; ammonia; asolution containing hydrofluoric acid (HF), nitric acid (HNO₃), and/oracetic acid (CH₃COOH); or other suitable wet etchant. Numerous otherembodiments of methods to form the fins 218 may be suitable.

Still referring to FIG. 4B, isolation features 230 surround a bottomportion of fins 218 to separate and isolate fins 218 from each other.Isolation features 230 include silicon oxide, silicon nitride, siliconoxynitride, other suitable isolation material (for example, includingsilicon, oxygen, nitrogen, carbon, or other suitable isolationconstituent), or combinations thereof. Isolation features 230 caninclude different structures, such as shallow trench isolation (STI)structures and/or deep trench isolation (DTI) structures. In anembodiment, the isolation features 230 can be formed by filling thetrenches between fins 218 with insulator material(s) (for example, byusing a CVD process or a spin-on glass process), performing a chemicalmechanical polishing (CMP) process to remove excessive insulatormaterial and/or planarize a top surface of the insulator material layer,and etching back the insulator material layer to form isolation features230. In some embodiments, isolation features 230 include a multi-layerstructure, such as a silicon nitride layer disposed over a thermal oxideliner layer. In some embodiments, the device 200 may form otherisolation features (or structures) such as dielectric fins (not shown)over the isolation features 230 and arranged in parallel with the fins218. The dielectric fins may include low-k dielectric material(s),high-k dielectric material(s), or a mix of low-k and high-k dielectricmaterials. Low-k dielectric material generally refers to dielectricmaterials having a low dielectric constant, for example, lower than thatof silicon oxide (k≈3.9), while high-k dielectric material generallyrefers to dielectric materials having a high dielectric constant, forexample, higher than that of silicon oxide.

At operation 106, the method 100 (FIG. 1A) forms dummy (or sacrificial)gate stacks 240 over the fins 218 and the isolation features 230 andforms gate spacers 247 on sidewalls of the dummy gate stacks 240. Theresultant structure is shown in FIGS. 5A-5C according to an embodiment.FIG. 5A illustrates a top view of the device 200, and FIGS. 5B and 5Cillustrate cross-sectional views of the device 200, in portion, alongthe B-B line and the C-C line in FIG. 5A, respectively. The B-B line iscut into source/drain regions of the device 200. From a top view, thedummy gate stacks 240 are oriented lengthwise generally along the “y”direction perpendicular to the “x” direction. Dummy gate stacks 240 areformed by deposition processes, lithography processes, etchingprocesses, other suitable processes, or combinations thereof. Forexample, deposition processes are performed to form a dummy gatedielectric layer 235 and a dummy gate electrode layer 245 over the dummygate dielectric layer 235. In some embodiment, one or more hard masklayers 246 are deposited over the dummy gate electrode layer 245. Thedummy gate dielectric layer 235 may include a dielectric material, suchas silicon oxide, a high-k dielectric material, other suitabledielectric material. In some embodiments, the dummy gate electrode layer245 includes polysilicon or other suitable material and the one or morehard mask layers 246 include silicon oxide, silicon nitride, or othersuitable materials. The deposition process may include CVD, physicalvapor deposition (PVD), atomic layer deposition (ALD), high densityplasma CVD (HDPCVD), metal organic CVD (MOCVD), remote plasma CVD(RPCVD), plasma enhanced CVD (PECVD), low-pressure CVD (LPCVD), atomiclayer CVD (ALCVD), atmospheric pressure CVD (APCVD), other suitablemethods, or combinations thereof. A lithography patterning and etchingprocess is then performed to pattern the one or more hard mask layers246, the dummy gate electrode layer 245, and the dummy gate dielectriclayer 235 to form dummy gate stacks 240, as depicted in FIG. 5C. Thelithography patterning processes include resist coating (for example,spin-on coating), soft baking, mask aligning, exposure, post-exposurebaking, developing the resist, rinsing, drying (for example, hardbaking), other suitable lithography processes, or combinations thereof.The etching processes include dry etching processes, wet etchingprocesses, other etching methods, or combinations thereof.

In the present embodiment, gate spacers 247 are formed by any suitableprocess and include a dielectric material. The dielectric material caninclude silicon, oxygen, carbon, nitrogen, other suitable material, orcombinations thereof (e.g., silicon oxide, silicon nitride, siliconoxynitride (SiON), silicon carbide, silicon carbon nitride (SiCN),silicon oxycarbide (SiOC), silicon oxycarbon nitride (SiOCN)). Forexample, a dielectric layer including silicon and nitrogen, such as asilicon nitride layer, can be deposited over dummy gate stacks 240 andsubsequently etched (e.g., anisotropically etched) to form gate spacers247. In some embodiments, gate spacers 247 include a multi-layerstructure, such as a first dielectric layer that includes siliconnitride and a second dielectric layer that includes silicon oxide. Insome embodiments, more than one set of spacers, such as seal spacers,offset spacers, sacrificial spacers, dummy spacers, and/or main spacers,are formed adjacent to dummy gate stacks 240. In the present embodiment,the process of forming the gate spacers 247 also forms fin sidewallspacers 247′, and the fin sidewall spacers 247′ comprise the samematerial as the gate spacers 247. A height of the fin sidewall spacers247′ may be used for tuning a size and shape of source/drain features.In some embodiments, the fin sidewall spacers 247′ are omitted orremoved from the device 200.

At operation 108, the method 100 (FIG. 1A) forms source/drain (S/D)trenches 250 by etching the fins 218 adjacent the gate spacers 247 andthen form inner spacers 255. The resultant structure is shown in FIGS.6A-6C according to an embodiment. FIG. 6A illustrates a top view of thedevice 200, and FIGS. 6B and 6C illustrate cross-sectional views of thedevice 200, in portion, along the B-B line and the C-C line in FIG. 6A,respectively. Particularly, the B-B line is cut into the source/drainregions of the transistors and is parallel to the gate stacks 240. TheB-B lines in FIGS. 7A through 8A are similarly configured. In anembodiment, an etching process completely removes semiconductor layerstack 205 in source/drain regions of fins 218 thereby exposing thesubstrate portion 204 of fins 218 in the source/drain regions. In someembodiments, the etching process further removes some, but not all, ofthe substrate portion of fins 218, such that source/drain trenches 250extend below a topmost surface of substrate 201. The etching process caninclude a dry etching process, a wet etching process, other suitableetching process, or combinations thereof. In some embodiments, theetching process is a multi-step etch process. For example, the etchingprocess may alternate etchants to separately and alternately removesemiconductor layers 210/210′ and semiconductor layers 215/215′. In someembodiments, parameters of the etching process are configured toselectively etch semiconductor layer stack with minimal (to no) etchingof gate stacks 240 and/or isolation features 230. In some embodiments, alithography process, such as those described herein, is performed toform a patterned mask layer that covers gate stacks 240 and/or isolationfeatures 230, and the etching process uses the patterned mask layer asan etch mask. In an embodiment, the fin sidewall spacers 247′ (ifpresent) may be recessed as well, as illustrated in FIG. 6B.

The operation 108 further forms inner spacers 255 (see FIG. 6C) alongsidewalls of semiconductor layers 210/210′ inside the S/D trenches 250.For example, a first etching process is performed that selectivelyetches semiconductor layers 210/210′ exposed by source/drain trenches250 with minimal (to no) etching of semiconductor layers 215/215′, suchthat gaps are formed between semiconductor layers 215/215′ and betweensemiconductor layer 215′ and semiconductor layer 204 under gate spacers247. Portions (edges) of semiconductor layers 215/215′ are thussuspended in the channel regions under gate spacers 247. In someembodiments, the gaps extend partially under dummy gate stacks 240. Thefirst etching process is configured to laterally etch (e.g., along the“x” direction) semiconductor layers 210 and 210′, thereby reducing alength of semiconductor layers 210 and 210′ along the “x” direction. Thefirst etching process is a dry etching process, a wet etching process,other suitable etching process, or combinations thereof. A depositionprocess then forms a spacer layer over gate structures 240 and overfeatures defining source/drain trenches 250 (e.g., semiconductor layers215/215′, 210/210′, and 204), such as CVD, PVD, ALD, HDPCVD, MOCVD,RPCVD, PECVD, LPCVD, ALCVD, APCVD, other suitable methods, orcombinations thereof. The spacer layer partially (and, in someembodiments, completely) fills the source/drain trenches 250. Thedeposition process is configured to ensure that the spacer layer fillsthe gaps between semiconductor layers 215/215′ and between semiconductorlayer 215′ and semiconductor layer 204 under gate spacers 247. A secondetching process is then performed that selectively etches the spacerlayer to form inner spacers 255 as depicted in FIG. 6C with minimal (tono) etching of semiconductor layers 215/215′, dummy gate stacks 240, andgate spacers 247. In some embodiments, the spacer layer is removed fromsidewalls of gate spacers 247, sidewalls of semiconductor layers215/215′, dummy gate stacks 240, and semiconductor layer 204. The spacerlayer (and thus inner spacers 255) includes a material that is differentthan a material of semiconductor layers 215/215′/204 and a material ofgate spacers 247 to achieve desired etching selectivity during thesecond etching process. In some embodiments, the spacer layer 255includes a dielectric material that includes silicon, oxygen, carbon,nitrogen, other suitable material, or combinations thereof (for example,silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, orsilicon oxycarbonitride). In some embodiments, the inner spacer layer255 includes a low-k dielectric material, such as those describedherein.

At operation 110, the method 100 (FIG. 1A) epitaxially growssemiconductor S/D features 260 in the S/D trenches 250. The resultantstructure is shown in FIGS. 7A-7C according to an embodiment. FIG. 7Aillustrates a top view of the device 200, and FIGS. 7B and 7C illustratecross-sectional views of the device 200, in portion, along the B-B lineand the C-C line in FIG. 7A, respectively. As shown in FIGS. 7B and 7C,epitaxial S/D features 260 are grown from the semiconductor layer 204 atthe bottom of the S/D trenches 250 and from the semiconductor layers 215and 215′ at the sidewalls of the S/D trenches 250. An epitaxy processcan use CVD deposition techniques (for example, VPE and/or UHV-CVD),molecular beam epitaxy, other suitable epitaxial growth processes, orcombinations thereof. The epitaxy process can use gaseous and/or liquidprecursors, which interact with the composition of the semiconductorlayers 204, 215, and 215′. Epitaxial S/D features 260 are doped withn-type dopants or p-type dopants for n-type transistors or p-typetransistors respectively. In some embodiments, for n-type transistors,epitaxial S/D features 260 include silicon and can be doped with carbon,phosphorous, arsenic, other n-type dopant, or combinations thereof (forexample, forming Si:C epitaxial source/drain features, Si:P epitaxialsource/drain features, or Si:C:P epitaxial source/drain features). Insome embodiments, for p-type transistors, epitaxial S/D features 260include silicon germanium or germanium and can be doped with boron,other p-type dopant, or combinations thereof (for example, formingSi:Ge:B epitaxial source/drain features). In some embodiments, epitaxialS/D features 260 include more than one epitaxial semiconductor layer,where the epitaxial semiconductor layers can include the same ordifferent materials and/or dopant concentrations. In some embodiments,epitaxial S/D features 260 include materials and/or dopants that achievedesired tensile stress and/or compressive stress in respective channelregions. In some embodiments, epitaxial source/drain features 260 aredoped during deposition by adding impurities to a source material of theepitaxy process (i.e., in-situ). In some embodiments, epitaxialsource/drain features 260 are doped by an ion implantation processsubsequent to a deposition process. In some embodiments, annealingprocesses (e.g., rapid thermal annealing (RTA) and/or laser annealing)are performed to activate dopants in epitaxial source/drain features260. In some embodiments, epitaxial source/drain features 260 are formedin separate processing sequences that include, for example, maskingp-type GAA transistor regions when forming epitaxial source/drainfeatures 260 in n-type GAA transistor regions and masking n-type GAAtransistor regions when forming epitaxial source/drain features 260 inp-type GAA transistor regions.

At operation 112, the method 100 (FIG. 1A) forms a contact etch stoplayer (CESL) 269 and an inter-layer dielectric (ILD) layer 270. Theresultant structure is shown in FIGS. 8A-8C according to an embodiment.FIG. 8A illustrates a top view of the device 200, and FIGS. 8B and 8Cillustrate cross-sectional views of the device 200, in portion, alongthe B-B line and the C-C line in FIG. 8A, respectively. The CESL 269 isdeposited over the S/D features 260, the isolation features 230, and thefin sidewall spacers 247′ (if present). The ILD layer 270 is depositedover the CESL 269 and fills the space between opposing gate spacers 247and between the S/D features 260. The CESL 269 includes a material thatis different than ILD layer 270. The CESL 269 may include La₂O₃, Al₂O₃,SiOCN, SiOC, SiCN, SiO₂, SiC, ZnO, ZrN, Zr₂Al₃O₉, TiO₂, TaO₂, ZrO₂,HfO₂, Si₃N₄, Y₂O₃, AlON, TaCN, ZrSi, or other suitable material(s); andmay be formed by CVD, PVD, ALD, or other suitable methods. The ILD layer270 may comprise tetraethylorthosilicate (TEOS) oxide, un-doped silicateglass, or doped silicon oxide such as borophosphosilicate glass (BPSG),fluoride-doped silica glass (FSG), phosphosilicate glass (PSG), borondoped silicon glass (BSG), a low-k dielectric material, other suitabledielectric material, or combinations thereof. The ILD 270 may be formedby PECVD (plasma enhanced CVD), FCVD (flowable CVD), or other suitablemethods. Subsequent to the deposition of the CESL 269 and the ILD layer270, a chemical mechanical planarization (CMP) process and/or otherplanarization process is performed until reaching a top portion of dummygate stacks 240. In some embodiments, the planarization process removeshard mask layers 246 of dummy gate stacks 240 to expose underlying dummygate electrodes 245, such as polysilicon gate electrode layers.

At operation 114, the method 100 (FIG. 1A) removes the dummy gate stacks240 to form gate trenches 275. The resultant structure is shown in FIGS.9A-9C according to an embodiment. FIG. 9A illustrates a top view of thedevice 200, and FIGS. 9B and 9C illustrate cross-sectional views of thedevice 200, in portion, along the B-B line and the C-C line in FIG. 9A,respectively. Particularly, the B-B line in FIG. 9A cuts into the device200 in the channel region (or gate region). The operation 114 may useone or more dry etching processes, wet etching processes, other suitableetching processes, or combinations thereof. In some embodiments, theetching process is a multi-step etch process. For example, the etchingprocess may alternate etchants to separately remove various layers ofdummy gate stacks 240. In some embodiments, the etching process isconfigured to selectively etch dummy gate stacks 240 with minimal (tono) etching of other features of the device 200, such as ILD layer 270,gate spacers 247, isolation features 230, semiconductor layers 215 and215′, and semiconductor layers 210 and 210′. As a result, thesemiconductor layers 215 and 215′, the semiconductor layers 210 and210′, the inner spacers 255, the semiconductor layer 204, and theisolation features 230 are exposed in the gate trenches 275.

At operation 116, the method 100 (FIG. 1A) removes the semiconductorlayers 210 and 210′ exposed in the gate trenches 275, leaving thesemiconductor layers 215 and 215′ suspended over the semiconductor layer204 and connected with the S/D features 260. The resultant structure isshown in FIGS. 10A-10C according to an embodiment. FIG. 10A illustratesa top view of the device 200, and FIGS. 10B and 10C illustratecross-sectional views of the device 200, in portion, along the B-B lineand the C-C line in FIG. 10A, respectively. Particularly, the B-B linein FIG. 10A cuts into the device 200 in the channel region (or gateregion). This process is also referred to as a channel release processand the semiconductor layers 215 and 215′ are also referred to aschannel layers. The etching process selectively etches semiconductorlayers 210 and 210′ with minimal (to no) etching of semiconductor layers215 and 215′ and, in some embodiments, minimal (to no) etching of gatespacers 247 and/or inner spacers 255.

At operation 118, the method 100 (FIG. 1A) forms an interfacial layer280 over the surfaces of the semiconductor layers 215 and 215′ that areexposed in the gate trenches 275. The resultant structure is shown inFIG. 11 according to an embodiment. FIG. 11 illustrates across-sectional view of the device 200, in portion, along the B-B linein FIG. 10A. For purpose of simplicity, not all features of the device200 are shown in FIG. 11 . In some embodiments, the interfacial layer280 is formed by an oxidation process such as thermal oxidation orchemical oxidation by oxidizing surfaces of the semiconductor layers215, 215′, and 204 that are exposed in the gate trenches 275. In thoseembodiments, the interfacial layer 280 is not formed over the surfacesof the isolation features 230. In some embodiments, the interfaciallayer 280 is formed by a deposition process such as ALD or CVD oversurfaces of the semiconductor layers 215, 215′, and 204 and theisolation features 230 that are exposed in the gate trenches 275, suchas depicted in FIG. 11 . The interfacial layer 280 may include silicondioxide, silicon oxynitride, or other suitable materials. Theinterfacial layer 280 is formed to a thickness t5. In the presentembodiment, t5 is less than half of S2. For example, t5 may be about 0.5nm to about 1 nm in some embodiments. FIG. 11 also illustrates variousdimensions of the device 200 at this fabrication stage. The thicknessest1′ and t3′ of the semiconductors 215 and 215′, respectively, may besubstantially the same as the thicknesses t1 and t3 discussed withreference to FIG. 3 , with differences between t1 and t1′ and between t3and t3′ caused by the channel release process and the oxidation processif that is used for forming the interfacial layer 280. In someembodiments, the thickness t1′ may be in a range of about 4 nm to about6 nm, the thicknesses t3′ may be in a range of about 1 nm to about 4 nm,while it is maintained that t3′ is smaller than t1′. In someembodiments, a ratio of t3′ to t1′ is in a range of about 0.33 to about1.0 and less than 1.0. These disclosed ranges are designed to reduceleakage current (from sub-channel effects) through the semiconductorlayer 215′ and to improve channel uniformity among the channel layers215, as discussed above with reference to FIG. 3 . The vertical spacingS1 between adjacent layers 215 and between layer 215 and layer 215′ isabout the same as the thickness t2 (FIG. 3 ) and the vertical spacing S2between layer 215′ and layer 204 is about the same as the thickness t4(FIG. 3 ), with differences between S1 and t2 and between S2 and t4caused by the channel release process and the oxidation process if thatis used for forming the interfacial layer 280. In some embodiment, S1 isgreater than S2 by about 3 nm to about 6 nm. In some embodiments, S2 isabout 2 nm to about 5 nm, while S1 is greater than S2 by about 3 nm toabout 6 nm. In some embodiments, a ratio of S2 to S1 is designed to bein a range of about 0.33 to about 0.6. The importance of these dimensionand ratio ranges for S2 and S1 is the same as that discussed above forthicknesses t4 and t2 in FIG. 3 . For example, if the ratio of S2 to S1is too small (such as less than 0.33) or too large (such as more than0.6) or if S2 is too small (less than 2 nm) or too large (more than 5nm), it might be difficult to fully fill the space between the layers215′ and 204 with dielectric material(s) or the dielectric materialstherein might not provide sufficient isolation for TDDB performancepurposes.

Next, the method 100 proceeds to forming one or more dielectricmaterials to fully fill the space between the layer 215′ and the layer204 and then form a high-k metal gate 240′ over the channel layers 215and 215′. FIGS. 1B and 1C illustrate two alternative embodiments of themethod 100 for the above purposes. FIG. 12A, 12B, and 12C illustratecross-sectional views of the device 200, in portion, along the B-B linein FIG. 10A, according to the method 100 in FIG. 1B. FIG. 13A, 13B, 13C,and 13D illustrate cross-sectional views of the device 200, in portion,along the B-B line in FIG. 10A, according to the method 100 in FIG. 1C.FIGS. 1B and 1C are separately discussed below.

Referring to FIG. 1B, the method 100 proceeds from the operation 118 tooperation 120 to form a high-k dielectric layer 281 over the interfaciallayer 280. A resultant structure of the device 200 is shown in FIG. 12A.In the present embodiment, the high-k dielectric layer 281 surrounds theinterfacial layer 280 that in turn surrounds the semiconductor layers215 and 215′. The high-k dielectric layer 281 is also disposed over theinterfacial layer 280 that is over the semiconductor layer 204 and theisolation features 230. The dielectric layer 281 may include a high-kdielectric material such as HfO₂, HfSiO, HfSiO₄, HfSiON, HfLaO, HfTaO,HfTiO, HfZrO, HfAlOx, ZrO, ZrO₂, ZrSiO₂, AlO, AlSiO, Al₂O₃, TiO, TiO₂,LaO, LaSiO, Ta₂O₃, Ta₂O₅, Y₂O₃, SrTiO₃, BaZrO, BaTiO₃ (BTO), (Ba,Sr)TiO₃(BST), Si₃N₄, hafnium dioxide-alumina (HfO₂-Al₂O₃) alloy, other suitablehigh-k dielectric material, or combinations thereof. High-k dielectricmaterial generally refers to dielectric materials having a highdielectric constant, for example, greater than that of silicon oxide(k≈3.9). The dielectric layer 281 may be formed by chemical oxidation,thermal oxidation, ALD, CVD, and/or other suitable methods. Thedielectric layer 281 is formed to a thickness t6. In the presentembodiment, the thickness t6 is controlled such that t6 is less thanhalf of (S1−2*t5) but greater than or equal to half of (S2−2*t5). Whenthe thickness t6 is controlled to be in such range, the dielectric layer281 disposed on the bottom surface of the layer 215′ and on the topsurface of the layer 204 merge into one dielectric layer and filling thespace between the layers 215′ and 204, while the dielectric layer 281disposed over the semiconductor layers 215 do not merge with each otherand the dielectric layer 281 disposed over the semiconductor layers 215and 215′ do not merge with each other.

Referring to FIG. 1B, at operation 122, the method 100 etches thedielectric layer 281 so that only the merged portion of the dielectriclayer 281 between the layers 215′ and 204 remains in the device 200while the rest of the dielectric layer 281 is removed. Referring to FIG.12B, the dielectric layer 281 is removed from the area surrounding thesemiconductor layers 215, above the semiconductor layer 215′, and abovethe isolation features 230. In an embodiment, operation 122 applies anisotropic etching process to the dielectric layer 281. Further, theisotropic etching process is tuned selective to the material(s) in thedielectric layer 281 with no (or minimal) etching to the interfaciallayer 280, the isolation features 230, the semiconductor layers 215,215′, and 204, as well as other features exposed in the gate trenches275 including the inner spacers 255 and the gate spacers 247 (FIGS. 10Band 10C). The isotropic etching process may use an etchant such as SPMcleaning solution (a mixture of H₂SO₄ and H₂O₂ with a ratio ofH₂SO₄:H₂O₂ of 1:4 for example), diluted hydro fluoride acid (dHF, amixture of hydro fluoride and water), or other suitable etchant(s). Theetchant is applied into the space between adjacent semiconductor layers215/215′ and to completely remove the dielectric layer 281 from the areasurrounding the semiconductor layers 215 and above the semiconductorlayer 215′. The etchant also laterally recesses the dielectric layer 281between the semiconductor layers 215′ and 204, but a substantial portionof the dielectric layer 281 remains between the semiconductor layers215′ and 204. As a result, a substantial portion of the space betweenthe semiconductor layers 215′ and 204 remains filled by a dielectricfeature 285 that includes the dielectric layer 281 sandwiched by theinterfacial layer 280. Further, the dielectric feature 285 is (center)aligned with the layer 215′ and the semiconductor fin 204. Thus, thedielectric feature 285 is a self-aligned dielectric capping layer. Insome embodiments, other portions of the interfacial layer 280 may bepartially or completely consumed by the isotropic etching process.

Referring to FIG. 1B, at operation 124, the method 100 repairs theinterfacial layer 280. For example, operation 124 may perform a cleaningprocess, a thermal process, a deposition process, or other suitableprocesses to re-form the interfacial layer 280 or to add thickness tothe interfacial layer 280. In some embodiments of the method 100, theoperation 124 is optional and may be skipped or omitted.

Referring to FIG. 1B, at operation 126, the method 100 forms a high-kmetal gate 240′ engaging the channel layers 215 and 215′, such as shownin FIG. 12C according to an embodiment. For example, the operation 126forms a high-k dielectric layer 349 over the interfacial layer 280 andthe high-k dielectric layer 281, forms one or more work function metallayers 340 over the high-k dielectric layer 349, and forms a gateelectrode 350 over the work function metal layers 340. In someembodiments, the interfacial layer 280 is considered part of the high-kmetal gate 240′. The high-k dielectric layer 349 may include a materialthat is the same as or different from the material in the high-kdielectric layer 281. The high-k dielectric layer 349 may include HfO₂,HfSiO, HfSiO₄, HfSiON, HfLaO, HfTaO, HfTiO, HfZrO, HfAlOx, ZrO, ZrO₂,ZrSiO₂, AlO, AlSiO, Al₂O₃, TiO, TiO₂, LaO, LaSiO, Ta₂O₃, Ta₂O₅, Y₂O₃,SrTiO₃, BaZrO, BaTiO₃ (BTO), (Ba,Sr)TiO₃ (BST), Si₃N₄, hafniumdioxide-alumina (HfO₂-Al₂O₃) alloy, other suitable high-k dielectricmaterial, or combinations thereof. The high-k dielectric layer 349 maybe formed by chemical oxidation, thermal oxidation, atomic layerdeposition (ALD), chemical vapor deposition (CVD), and/or other suitablemethods. The work function metal layer 340 may be an n-type or a p-typework function layer, depending on the type of the GAA transistor. Ann-type work function layer may comprise a metal with sufficiently loweffective work function such as titanium, aluminum, tantalum carbide,tantalum carbide nitride, tantalum silicon nitride, or combinationsthereof. A p-type work function layer may comprise a metal with asufficiently large effective work function, such as titanium nitride,tantalum nitride, ruthenium, molybdenum, tungsten, platinum, orcombinations thereof. The gate electrode layer 350 may include aluminum,tungsten, cobalt, copper, and/or other suitable materials. The gateelectrode layer 350 may be formed by CVD, PVD, plating, and/or othersuitable processes. After the semiconductor layer 204 (or a portionthereof) is replaced with a backside via as will be discussed, thedielectric layers 280, 281, and 349 collectively isolate the backsidevia from the metal gate 240′.

Referring to FIG. 1C (an alternative embodiment to FIG. 1B), the method100 proceeds from the operation 118 to operation 121 to form a high-kdielectric layer 287 over the interfacial layer 280. A resultantstructure of the device 200 is shown in FIG. 13A. In the presentembodiment, the high-k dielectric layer 287 surrounds the interfaciallayer 280 that in turn surrounds the semiconductor layers 215 and 215′.The high-k dielectric layer 287 is also disposed over the interfaciallayer 280 that is over the semiconductor layer 204 and the isolationfeatures 230. The dielectric layer 287 may include a high-k dielectricmaterial such as HfO₂, HfSiO, HfSiO₄, HfSiON, HfLaO, HfTaO, HfTiO,HfZrO, HfAlOx, ZrO, ZrO₂, ZrSiO₂, AlO, AlSiO, Al₂O₃, TiO, TiO₂, LaO,LaSiO, Ta₂O₃, Ta₂O₅, Y₂O₃, SrTiO₃, BaZrO, BaTiO₃ (BTO), (Ba,Sr)TiO₃(BST), Si₃N₄, hafnium dioxide-alumina (HfO₂-Al₂O₃) alloy, other suitablehigh-k dielectric material, or combinations thereof. The dielectriclayer 287 may be formed by chemical oxidation, thermal oxidation, ALD,CVD, and/or other suitable methods. The dielectric layer 287 is formedto a thickness t7. In the present embodiment, the thickness t7 iscontrolled to be less than half of (S2−2*t5) so that the dielectriclayer 287 does not fill the space between the layers 215′ and 204 anddoes not fill the space between the layers 215 and 215′. In someembodiments, t7 may be in a range of about 0.8 nm to about 1.2 nm.

Referring to FIG. 1C, at operation 123, the method 100 forms a low-kdielectric layer 283 over the high-k dielectric layer 287. A resultantstructure of the device 200 is shown in FIG. 13B. In the presentembodiment, the low-k dielectric layer 283 surrounds the high-kdielectric layer 287. The low-k dielectric layer 283 is also disposedover the high-k dielectric layer 287 that is over the semiconductorlayer 204 and the isolation features 230. The low-k dielectric layer 283may include a low-k dielectric material such as a dielectric materialincluding Si, O, N, and C (for example, SiOCN, SiOC, SiCN, SiO₂, Si₃N₄,or a combination thereof), other suitable low-k dielectric material, orcombinations thereof. Exemplary low-k dielectric materials include FSG,carbon doped silicon oxide, Black Diamond® (Applied Materials of SantaClara, California), Xerogel, Aerogel, amorphous fluorinated carbon,Parylene, BCB, SILK (Dow Chemical, Midland, Michigan), polyimide, orcombinations thereof. The low-k dielectric layer 283 may be formed bychemical oxidation, thermal oxidation, ALD, CVD, and/or other suitablemethods. The dielectric layer 283 is formed to a thickness t8. In thepresent embodiment, the thickness t8 is controlled such that t8 is lessthan half of (S1−2*t5−2*t7) but greater than or equal to half of(S2−2*t5−2*t7). When the thickness t8 is controlled to be in such range,the dielectric layer 283 disposed on the bottom surface of the layer215′ and on the top surface of the layer 204 merge into one dielectriclayer and filling the space between the layers 215′ and 204, while thedielectric layer 283 disposed over the semiconductor layers 215 does notmerge with each other and the dielectric layer 283 disposed over thesemiconductor layers 215 and 215′ does not merge with each other.

Referring to FIG. 1C, at operation 125, the method 100 etches thedielectric layer 283 so that only the merged portion of the dielectriclayer 283 between the layers 215′ and 204 remains in the device 200while the rest of the dielectric layer 283 is removed. Referring to FIG.13C, the dielectric layer 283 is removed from the area surrounding thesemiconductor layers 215, above the semiconductor layer 215′, and abovethe isolation features 230. In an embodiment, operation 125 applies anisotropic etching process to the dielectric layer 283. Further, theisotropic etching process is tuned selective to the material(s) in thedielectric layer 283 with no (or minimal) etching to the high-kdielectric layer 287 and other features exposed in the gate trenches 275including the inner spacers 255 and the gate spacers 247 (FIGS. 10B and10C). The isotropic etching process may use a dry etching method or awet etching method. For example, the isotropic etching process may usean etchant such as SPM (a mixture of H₂SO₄ and H₂O₂), DHF (a mixture ofHF and H₂O), BCl₃, HBr, chlorine, or other suitable etchant(s). Theetchant is applied into the space between adjacent semiconductor layers215/215′ and to completely remove the dielectric layer 283 from the areasurrounding the semiconductor layers 215 and above the semiconductorlayer 215′. The etchant also laterally recesses the dielectric layer 283between the semiconductor layers 215′ and 204, but a substantial portionof the dielectric layer 283 remains between the semiconductor layers215′ and 204. As a result, a substantial portion of the space betweenthe semiconductor layers 215′ and 204 remains filled by a dielectricfeature 289 that includes the dielectric layer 283 sandwiched by thehigh-k dielectric layer 287 and the interfacial layer 280. Further, thedielectric feature 289 is (center) aligned with the layer 215′ and thesemiconductor fin 204. Thus, the dielectric feature 289 is aself-aligned dielectric capping layer. In the present embodiment, thehigh-k dielectric layer 287 protects the interfacial layer 280 from theisotropic etching process.

Referring to FIG. 1C, at operation 127, the method 100 forms a high-kmetal gate 240′ engaging the channel layers 215 and 215′, such as shownin FIG. 13D according to an embodiment. For example, the operation 127forms a high-k dielectric layer 349 over the high-k dielectric layer 287and the low-k dielectric layer 283, forms one or more work functionmetal layers 340 over the high-k dielectric layer 349, and forms a gateelectrode 350 over the work function metal layers 340. In someembodiments, the interfacial layer 280 is considered part of the high-kmetal gate 240′. The high-k dielectric layer 349 may include a materialthat is the same as or different from the material in the high-kdielectric layer 287. The high-k dielectric layer 349 may include HfO₂,HfSiO, HfSiO₄, HfSiON, HfLaO, HfTaO, HfTiO, HfZrO, HfAlOx, ZrO, ZrO₂,ZrSiO₂, AlO, AlSiO, Al₂O₃, TiO, TiO₂, LaO, LaSiO, Ta₂O₃, Ta₂O₅, Y₂O₃,SrTiO₃, BaZrO, BaTiO₃ (BTO), (Ba,Sr)TiO₃ (BST), Si₃N₄, hafniumdioxide-alumina (HfO₂-Al₂O₃) alloy, other suitable high-k dielectricmaterial, or combinations thereof. The high-k dielectric layer 349 maybe formed by chemical oxidation, thermal oxidation, atomic layerdeposition (ALD), chemical vapor deposition (CVD), and/or other suitablemethods. The work function metal layer 340 may be an n-type or a p-typework function layer, depending on the type of the GAA transistor. Forexample, an n-type work function layer may comprise a metal withsufficiently low effective work function such as titanium, aluminum,tantalum carbide, tantalum carbide nitride, tantalum silicon nitride, orcombinations thereof. For example, a p-type work function layer maycomprise a metal with a sufficiently large effective work function, suchas titanium nitride, tantalum nitride, ruthenium, molybdenum, tungsten,platinum, or combinations thereof. The gate electrode layer 350 mayinclude aluminum, tungsten, cobalt, copper, and/or other suitablematerials. The gate electrode layer 350 may be formed by CVD, PVD,plating, and/or other suitable processes. As shown in FIG. 13D, thelow-k dielectric layer 283 is surrounded (or sealed off) by the high-kdielectric layers 287 and 349. After the semiconductor layer 204 (or aportion thereof) is replaced with a backside via as will be discussed,the low-k dielectric layer 283 functions to reduce the couplingcapacitance between the backside via and the metal gate 240′, while thedielectric layers 280, 283, 287, and 349 collectively isolate thebackside via from the metal gate 240′.

From either the operation 126 (FIG. 1B) or the operation 127 (FIG. 1C),the method 100 proceeds to operation 128 (FIG. 1D) to performmid-end-of-line (MEOL) processes and back-end-of-line (BEOL) processesat the frontside of the device 200. A resultant structure is shown inFIG. 14 according to an embodiment. For example, the operation 128 mayetch S/D contact holes exposing surfaces of some of the S/D features260, form S/D silicide features (not shown) on surfaces of the S/Dfeatures 260 exposed in the S/D contact holes, form S/D contacts (notshown) over the S/D silicide features, form gate vias 359 connecting tothe metal gates 240′, form S/D contact vias (not shown) connecting tothe S/D contacts, and form one or more interconnect layers with wiresand vias embedded in dielectric layers. The one or more interconnectlayers connecting gate, source, and drain electrodes of varioustransistors, as well as other circuits in the device 200, to form anintegrated circuit in part or in whole. The operation 128 may also formpassivation layer(s) over the interconnect layers. In the example shownin FIG. 14 , a layer 277 is used to denote various dielectric and metallayers including S/D contacts, S/D vias, gate vias, interconnect layers,and passivation layers formed at the frontside of the device 200.

At operation 130, the method 100 (FIG. 1D) attaches the frontside of thedevice 200 to a carrier 370, such as shown in FIG. 15 . The operation130 may use any suitable attaching processes, such as direct bonding,hybrid bonding, using adhesive, or other bonding methods. The operation130 may further include alignment, annealing, and/or other processes.The carrier 370 may be a silicon wafer in some embodiment.

At operation 132, the method 100 (FIG. 1D) flips the device 200 upsidedown, such as shown in FIG. 16 . This makes the device 200 accessiblefrom the backside of the device 200 for further processing. In thefigures of the present disclosure, the “z” direction points from thebackside of the device 200 to the frontside of the device 200, while the“−z” direction points from the frontside of the device 200 to thebackside of the device 200. The operation 132 then thins down the device200 from its backside until the semiconductor layer 204 and theisolation features 230 are exposed from the backside of the device 200.The resultant structure is shown in FIG. 17 according to an embodiment.The thinning process may include a mechanical grinding process and/or achemical thinning process. A substantial amount of substrate materialmay be first removed from the substrate 201 during a mechanical grindingprocess. Afterwards, a chemical thinning process may apply an etchingchemical to the backside of the substrate 201 to further thin down thesubstrate 201.

At operation 134, the method 100 (FIG. 1D) replaces the semiconductorfins 204 with dielectric fins 279. The resultant structure is shown inFIG. 18 according to an embodiment. Each dielectric fin 279 includes adielectric liner 274 and a dielectric filler 276 over the dielectricliner 274. In an embodiment, operation 134 includes selectively removingthe semiconductor fins 204 to form trenches and then depositing thedielectric liner 274 and the dielectric filler 276 to fill the trenches.In an embodiment, the operation 134 applies an etching process that istuned to be selective to the materials of the semiconductor fins 204(such as Si in an embodiment) and with no (or minimal) etching to theS/D features 260, the dielectric features 285 (or the dielectricfeatures 289 in some embodiments), the isolation features 230, the innerspacers 255, and the fin sidewall spacers 247′ if present. The etchingprocess can be dry etching, wet etching, reactive ion etching, or otheretching methods. Removing of the semiconductor fins 204 results intrenches. Then, the operation 134 deposits the dielectric liner 274 andthe dielectric filler 276 to fill the trenches. In an embodiment, thedielectric liner 274 includes silicon nitride and the dielectriclayer(s) 276 includes silicon oxide. In some embodiments, the dielectricliner 274 includes other dielectric materials such as La₂O₃, Al₂O₃,SiOCN, SiOC, SiCN, SiO₂, SiC, ZnO, ZrN, Zr₂Al₃O₉, TiO₂, TaO₂, ZrO₂,HfO₂, Y₂O₃, AlON, TaCN, ZrSi, or other suitable material(s). Thedielectric liner 274 may have a substantially uniform thickness alongthe various surfaces of the trenches, and may be formed by CVD, PVD,ALD, or other suitable methods. In some embodiments, the dielectriclayer(s) 276 may comprise tetraethylorthosilicate (TEOS) oxide, un-dopedsilicate glass, or doped silicon oxide such as borophosphosilicate glass(BPSG), fluoride-doped silica glass (FSG), phosphosilicate glass (PSG),boron doped silicon glass (BSG), and/or other suitable dielectricmaterials. The dielectric layer(s) 276 may be formed by PECVD (plasmaenhanced CVD), FCVD (flowable CVD), or other suitable methods.

In some embodiments, the operation 134 only replaces a part of thesemiconductor fins 204 with dielectric fins 279. For example, operation134 may form a patterned etch mask over the backside of the device 200.The patterned etch mask covers the area under the S/D features 260 thatare to be connected to backside vias and exposes the other area. Then,the operation 134 etches the semiconductor fins 204 through the etchmask to form trenches and deposits the dielectric layers 274 and 276 inthe trenches. Subsequently, operation 134 removes the etch mask. In suchembodiments, the semiconductor fins 204 are partially replaced with thedielectric fins 279. In some embodiments, the operation 134 is optionaland may be skipped or omitted in some embodiments of the method 100.

From either the operation 132 (if the operation 134 is skipped) or theoperation 134, the method 100 proceeds to operation 136 (FIG. 1D) toetch vias holes 278. The resultant structure is shown in FIGS. 19A-19Daccording to an embodiment. FIG. 19A illustrates a top view of thedevice 200, and FIGS. 19B, 19C, and 19D illustrate cross-sectional viewsof the device 200, in portion, along the B-B line, the C-C line, and theD-D line in FIG. 19A, respectively. As shown in FIG. 19B, the via hole278 penetrates through either the semiconductor layer 204 (if theoperation 134 is skipped) or the dielectric fin 276 (if the operation134 is performed) in alternative embodiments. Also, the via hole 278exposes portions of the dielectric features 285 (in embodiments whereoperations in FIG. 1B are included in the method 100) or the dielectricfeatures 289 (in embodiments where operations in FIG. 1C are included inthe method 100).

In the depicted embodiment, operation 136 forms a patterned etch mask360 over the backside of the device 200. The etch mask 360 exposes thearea under the S/D features 260 that are to be connected to backsidevias and covers the other area. In various embodiments, the etch mask360 may expose the backside of source features only, drain featuresonly, or both source and drain features. The etch mask 360 includes amaterial that is different than a material of the semiconductor fins 204(or the dielectric fin 276 in an alternative embodiment) to achieve etchselectivity. In an embodiment, the etch mask 360 includes a patternedresist. Alternatively, the etch mask 360 includes a patterned resistover a patterned hard mask. The present disclosure contemplates othermaterials for the etch mask 360, so long as etching selectivity isachieved during the etching of the semiconductor fins 204 or thedielectric fins 276. A patterned resist may be formed by a lithographyprocess that includes forming a resist layer, performing a pre-exposurebaking process, performing an exposure process, performing apost-exposure baking process, and performing a developing process. Then,operation 136 etches the semiconductor fins 204 (or the dielectric fin276 in an alternative embodiment) through the etch mask 360 to form thevia holes 278. The etching process may partially etch the S/D features260. The etching process results in via holes 278 that expose the S/Dfeatures 260 from the backside of the device 200. The etching processcan be dry etching, wet etching, reactive ion etching, or other etchingmethods. The dielectric features 285 (or the dielectric features 289 inan alternative embodiment) protects the metal gates 240′ from theetching process. Particularly, both the dielectric features 285 and thedielectric features 289 include high-k dielectric material(s), whichresist the etching process very well. Thus, the dielectric features 285and the dielectric features 289 provide good protection to the metalgates 240′. In some embodiments when the operation 134 replaces a partof the semiconductor fins 204, the operation 136 removes the rest of thesemiconductor fins 204 during the etching of the via holes 278.

At operation 138, the method 100 (FIG. 1C) forms a backside silicidefeature 261 and a backside S/D contact 282. The resultant structure isshown in FIGS. 20A-20D according to an embodiment. FIG. 20A illustratesa perspective view of the device 200, and FIGS. 20B, 20C, and 20Dillustrate cross-sectional views of the device 200, in portion, alongthe B-B line, the C-C line, and the D-D line in FIG. 20A, respectively.As illustrated in FIGS. 20B and 20D, the dielectric features 285 (or thedielectric features 289 in an alternative embodiment) isolate the metalgates 240′ from the silicide feature 261 and the S/D contact 282.Further, the low-k dielectric layer 283 in the dielectric features 289(FIG. 13D) helps reduce the coupling capacitance between the metal gates240′ and the S/D contact 282.

In an embodiment, the operation 138 includes depositing one or moremetals into the via holes 278, performing an annealing process to thedevice 200 to cause reaction between the one or more metals and the S/Dfeatures 260 to produce the silicide feature 261, and removingun-reacted portions of the one or more metals, leaving the silicidefeatures 261 in the via holes 278. The one or more metals may includetitanium (Ti), tantalum (Ta), tungsten (W), nickel (Ni), platinum (Pt),ytterbium (Yb), iridium (Ir), erbium (Er), cobalt (Co), or a combinationthereof (e.g., an alloy of two or more metals) and may be depositedusing CVD, PVD, ALD, or other suitable methods. The silicide feature 261may include titanium silicide (TiSi), nickel silicide (NiSi), tungstensilicide (WSi), nickel-platinum silicide (NiPtSi),nickel-platinum-germanium silicide (NiPtGeSi), nickel-germanium silicide(NiGeSi), ytterbium silicide (YbSi), platinum silicide (PtSi), iridiumsilicide (IrSi), erbium silicide (ErSi), cobalt silicide (CoSi), orother suitable compounds. In an embodiment, the contact 282 may includea conductive barrier layer and a metal fill layer over the conductivebarrier layer. The conductive barrier layer functions to prevent metalmaterials of the metal fill layer from diffusing into the dielectriclayers adjacent the contacts 282, such as the layers 230 and 276. Theconductive barrier layer may include titanium (Ti), tantalum (Ta),tungsten (W), cobalt (Co), ruthenium (Ru), or a conductive nitride suchas titanium nitride (TiN), titanium aluminum nitride (TiAlN), tungstennitride (WN), tantalum nitride (TaN), or combinations thereof, and maybe formed by CVD, PVD, ALD, and/or other suitable processes. The metalfill layer may include tungsten (W), cobalt (Co), molybdenum (Mo),ruthenium (Ru), copper (Cu), aluminum (Al), titanium (Ti), tantalum(Ta), or other metals, and may be formed by CVD, PVD, ALD, plating, orother suitable processes. In some embodiments, the conductive barrierlayer is omitted in the source contact 282. The operation 138 mayperform a CMP process to remove excessive materials of the sourcecontact 282. In some embodiments when the operation 134 is skipped, theoperation 138 may replace the rest of the semiconductor fins 204 withdielectric fins 279 after the vias 282 are deposited.

At operation 140, the method 100 (FIG. 1D) forms backside power rails284 and a backside interconnect 286. The resultant structure is shown inFIGS. 21A and 21B according to an embodiment. FIG. 21A illustrates aperspective view of the device 200, and FIG. 21B illustrates across-sectional view of the device 200, in portion, along the B-B linein FIG. 21A. As shown in FIG. 21B, the backside contacts 282 areelectrically connected to the backside power rails 284. In anembodiment, the backside power rails 284 may be formed using a damasceneprocess, a dual-damascene process, a metal patterning process, or othersuitable processes. The backside power rails 284 may include tungsten(W), cobalt (Co), molybdenum (Mo), ruthenium (Ru), copper (Cu), aluminum(Al), titanium (Ti), tantalum (Ta), or other metals, and may bedeposited by CVD, PVD, ALD, plating, or other suitable processes. Thebackside power rails 284 are embedded in one or more dielectric layers290. The backside interconnect 286 includes wires and vias embedded inone or more dielectric layers 290. The backside power rails 284 areconsidered part of the backside interconnect 286. Having backside powerrails 284 beneficially increases the number of metal tracks available inthe device 200 for directly connecting to source/drain contacts andvias. It also increases the gate density for greater device integrationthan other structures without the backside power rails 284. The backsidepower rails 284 may have wider dimension than the first level metal (MO)tracks on the frontside of the device 200, which beneficially reducesthe backside power rail resistance.

At operation 142, the method 100 (FIG. 1D) performs further fabricationprocesses to the device 200. For example, it may form passivation layerson the backside of the device 200, remove the carrier 370, and performother BEOL processes.

Although not intended to be limiting, embodiments of the presentdisclosure provide one or more of the following advantages. For example,embodiments of the present disclosure form a dielectric feature whichcomprises a high-k dielectric material or a low-k dielectric materialsurrounded by one or more high-k dielectric materials. The high-kdielectric material(s) provide good protection to metal gates duringbackside via hole etching processes. Also, the dielectric featureprovides good isolation between metal gates and backside vias. When thedielectric feature includes a low-k dielectric material surrounded byone or more high-k dielectric materials, it also reduces the couplingcapacitance between the metal gates and the backside vias. For anotherexample, embodiments of the present disclosure provide a stack ofchannel layers with different thicknesses. Particularly, the bottommostchannel layer is thinner than other channel layers, thereby reducingleakage due to sub-channel effects. Embodiments of the presentdisclosure can be readily integrated into existing semiconductormanufacturing processes.

In one example aspect, the present disclosure is directed to asemiconductor device that includes a first interconnect structure;multiple channel layers stacked over the first interconnect structure; agate stack wrapping around each of the channel layers except abottommost one of the channel layers; a source/drain feature adjoiningthe channel layers; a first conductive via connecting the firstinterconnect structure to a bottom of the source/drain feature; and adielectric feature between the bottommost one of the channel layers andthe first conductive via.

In some embodiments of the semiconductor device, the dielectric featureincludes one or more high-k dielectric materials. In a furtherembodiment, the dielectric feature includes a low-k dielectric materialsurrounded by the one or more high-k dielectric materials. In anotherfurther embodiment, the dielectric feature includes a semiconductoroxide layer between the one or more high-k dielectric materials and thefirst conductive via.

In an embodiment, the semiconductor device further includes asemiconductor fin structure directly below the channel layers and thesource/drain feature, wherein the first conductive via is embedded inthe semiconductor fin structure. In another embodiment, thesemiconductor device further includes a dielectric fin structuredirectly below the channel layers and the source/drain feature, whereinthe first conductive via is embedded in the dielectric fin structure. Inyet another embodiment, the semiconductor device further includes asecond interconnect structure above the channel layers, wherein thesecond interconnect structure includes a second conductive viaconnecting to a top of the gate stack.

In some embodiments of the semiconductor device, a first verticalspacing between adjacent ones of the channel layers is greater than athickness of the dielectric feature. In some embodiments, the bottommostone of the channel layers is thinner than other ones of the channellayers.

In another example aspect, the present disclosure is directed to amethod that includes providing a structure having a substrate, a firstlayer over the substrate, an isolation feature adjacent to sidewalls ofthe first layer, and channel layers over the first layer, wherein thechannel layers are spaced vertically away from each other by a firstspace, and a bottommost one of the channel layers is spaced verticallyaway from the first layer by a second space thinner than the firstspace. The method further includes forming an interfacial layer wrappingaround each of the channel layers and over the first layer and forming afirst high-k dielectric layer over the interfacial layer, wrappingaround each of the channel layers, and over the first layer, wherein theinterfacial layer and the first high-k dielectric layer collectivelycompletely fill the second space and only partially fill the firstspace. The method further includes etching the first high-k dielectriclayer to remove the first high-k dielectric layer from the first spaceand to keep a portion of the first high-k dielectric layer in the secondspace. After the etching of the first high-k dielectric layer, themethod further includes forming a second high-k dielectric layer overthe channel layers and the first layer and forming a gate electrode overthe second high-k dielectric layer.

In some embodiments of the method, the bottommost one of the channellayers is thinner than other ones of the channel layers. In someembodiments, before the forming of the second high-k dielectric layer,the method further includes repairing the interfacial layer. In someembodiments of the method, the second space is thinner than the firstspace by about 3 nm to 6 nm.

In some embodiments, the method further includes forming an interconnectstructure over the gate electrode; thinning down the substrate to exposethe first layer and the isolation feature; etching a via hole under thebottommost one of the channel layers; and forming a via structure in thevia hole, wherein the portion of the first high-k dielectric layerremains between the bottommost one of the channel layers and the viastructure. In a further embodiment, the method includes replacing atleast a portion of the first layer with a dielectric feature before theetching of the via hole.

In yet another example aspect, the present disclosure is directed to amethod that includes providing a structure having a substrate, a firstlayer over the substrate, an isolation feature adjacent to sidewalls ofthe first layer, and channel layers over the first layer, wherein thechannel layers are spaced vertically away from each other by a firstspace, and a bottommost one of the channel layers is spaced verticallyaway from the first layer by a second space thinner than the firstspace. The method further includes forming an interfacial layer wrappingaround each of the channel layers and over the first layer and forming afirst high-k dielectric layer over the interfacial layer, wrappingaround each of the channel layers, and over the first layer, wherein theinterfacial layer and the first high-k dielectric layer only partiallyfill the first space and only partially fill the second space. Themethod further includes forming a first low-k dielectric layer over thefirst high-k dielectric layer, wherein the interfacial layer, the firsthigh-k dielectric layer, and the first low-k dielectric layercollectively completely fill the second space and only partially fillthe first space. The method further includes etching the first low-kdielectric layer to remove the first low-k dielectric layer from thefirst space and to keep a portion of the first low-k dielectric layer inthe second space; forming a second high-k dielectric layer over thefirst high-k dielectric layer and the first low-k dielectric layer afterthe etching of the first low-k dielectric layer; and forming a gateelectrode over the second high-k dielectric layer.

In some embodiments of the method, the bottommost one of the channellayers is thinner than other ones of the channel layers. In someembodiments, the method further includes forming an interconnectstructure over the gate electrode; thinning down the substrate to exposethe first layer and the isolation feature; etching a via hole under thebottommost one of the channel layers; and forming a via structure in thevia hole, wherein the portion of the first high-k dielectric layer andthe first low-k dielectric layer remain between the bottommost one ofthe channel layers and the via structure. In a further embodiment, themethod includes replacing a portion of the first layer with a dielectricfeature before the etching of the via hole, wherein the via hole isetched through a remaining portion of the first layer. In anotherfurther embodiment, the structure further includes source/drain featuresover the first layer and connected by the channel layers, wherein thevia structure is electrically connected to one of the source/drainfeatures.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a firstinterconnect structure; multiple channel layers stacked over the firstinterconnect structure, wherein a bottommost one of the multiple channellayers is thinner than rest of the multiple channel layers; a gate stackwrapping around each of the channel layers except the bottommost one ofthe channel layers; a source/drain feature adjoining the channel layers;a first conductive via connecting the first interconnect structure to abottom of the source/drain feature; and a dielectric feature under thebottommost one of the channel layers and directly contacting the firstconductive via.
 2. The semiconductor device of claim 1, wherein thedielectric feature includes a high-k dielectric material.
 3. Thesemiconductor device of claim 2, wherein the dielectric feature furtherincludes a low-k dielectric material surrounded by the high-k dielectricmaterial.
 4. The semiconductor device of claim 1, further comprising:high-k gate dielectric layers wrapping around each of the channellayers, and the dielectric feature includes a same material as that ofthe high-k gate dielectric layers.
 5. The semiconductor device of claim1, further comprising: high-k gate dielectric layers wrapping aroundeach of the channel layers, and the dielectric feature includes adifferent material from that of the high-k gate dielectric layers. 6.The semiconductor device of claim 1, further comprising: a semiconductorfin structure directly below the channel layers and the source/drainfeature, wherein the first conductive via is embedded in thesemiconductor fin structure.
 7. The semiconductor device of claim 6,wherein the dielectric feature separates the semiconductor fin structurefrom the bottommost one of the channel layers.
 8. The semiconductordevice of claim 1, further comprising: a dielectric fin structuredirectly below the channel layers and the source/drain feature, whereinthe first conductive via is embedded in the dielectric fin structure. 9.The semiconductor device of claim 8, wherein the dielectric featureseparates the dielectric fin structure from the bottommost one of thechannel layers.
 10. The semiconductor device of claim 1, furthercomprising: inner spacers on sidewalls of the gate stack and verticallybetween the channel layers.
 11. A semiconductor device, comprising: afirst interconnect structure; multiple channel layers stacked over thefirst interconnect structure; a gate stack wrapping around each of thechannel layers except a bottommost one of the channel layers; innerspacers on sidewalls of the gate stack; a source/drain feature adjoiningthe channel layers and contacting the inner spacers; and a dielectricfeature under the bottommost one of the multiple channel layers andbetween two of the inner spacers.
 12. The semiconductor device of claim11, further comprising: a first conductive via connecting the firstinterconnect structure to a bottom of the source/drain feature.
 13. Thesemiconductor device of claim 12, wherein the dielectric featuredirectly contacts the first conductive via.
 14. The semiconductor deviceof claim 12, wherein a bottommost inner spacer directly contacts thefirst conductive via.
 15. The semiconductor device of claim 11, furthercomprising: high-k gate dielectric layers wrapping around each of thechannel layers, wherein a thickness of the dielectric feature is greaterthan a thickness of the high-k gate dielectric layers.
 16. Thesemiconductor device of claim 15, wherein the high-k gate dielectriclayers and the dielectric feature have different material compositions.17. A method, comprising: providing a structure having a substrate, afirst layer over the substrate, an isolation feature adjacent tosidewalls of the first layer, and channel layers over the first layer;forming a first high-k dielectric layer wrapping around each of thechannel layers, and over the first layer, wherein the first high-kdielectric layer partially fills a first space between the channellayers and completely fills a second space between the first layer and abottommost one of the channel layers; etching the first high-kdielectric layer to remove the first high-k dielectric layer from thefirst space and to keep a portion of the first high-k dielectric layerin the second space; after the etching of the first high-k dielectriclayer, forming a second high-k dielectric layer over the channel layersand the first layer; and forming a gate electrode over the second high-kdielectric layer.
 18. The method of claim 17, wherein the second spaceis thinner than the first space, and the first layer is thinner than thechannel layers.
 19. The method of claim 17, wherein the first high-kdielectric layer is thicker than the second high-k dielectric layer. 20.The method of claim 17, further comprising: forming an interconnectstructure over the gate electrode; thinning down the substrate to exposethe first layer and the isolation feature; etching a via hole under thebottommost one of the channel layers; and forming a via structure in thevia hole, wherein the portion of the first high-k dielectric layerremains between the bottommost one of the channel layers and the viastructure.